This invention concerns an apparatus and a method for checking value documents, in particular bank notes, and a value-document processing system.
In bank-note processing systems, properties of bank notes, such as printed image, denomination, authenticity and condition, are ascertained by capturing physical properties of the bank notes by means of sensors and evaluating the thereby generated sensor data.
Checking the condition, the so-called fitness, of a bank note involves checking whether it meets certain criteria to be able to be put back in circulation or have to be removed from circulation. Besides the degree of soiling and wear, an important criterion here is also the presence of unwanted foreign objects, usually in the form of adhesive tape or other stickers, on the bank note.
Checking the presence of adhesive tape is usually done by measuring the thickness of the bank note to be checked by means of mechanical or ultrasound thickness sensors.
From DE 10 2010 021 803 A1 it is further known to recognize adhesive tape by a dark-field transmission image of the bank note. This exploits the fact that adhesive tape extending beyond the edge of the bank note appears light in the transmission image, due to light scattering on the adhesive tape. When adhesive tape protrudes beyond the edge of the bank note, the value-document edge observed in transmission deviates from the usually straight edge course. Recognizing adhesive tape solely by the transmission image of the bank note is disadvantageous, however, since it can happen that distortions or deformations of the bank note, which can occur e.g. upon bank-note transport by machine and/or due to a cut edge not being completely straight, are falsely taken for adhesive tape, since with adhesive tape there is also observed a bank-note edge deviating from the straight edge course. A thus distorted or deformed bank note is then unnecessarily sorted out and possibly destroyed due to suspected adhesive tape, although it could have been put back in circulation.